a. Field of the Invention
The present invention relates generally to methods and apparatus for production of hydrogen, and, more particularly to methods and apparatus for producing hydrogen from an aluminum-based water-split reaction in a manner that is tailored to meet the requirements of particular equipment or applications, by differential distribution of one or more of the solid reactant materials in a matrix or other body such that areas of the differential distribution are contacted by the water in a sequential manner.
b. Related Art
As is well known, hydrogen gas has many different uses in a wide range of industries and activities. Perhaps the best known use is as a fuel, such as for combustion or use in a fuel cell, but there are many others, including lift gas for balloons or other lighter-than-air devices, use in certain types of welding, creation of artificial atmospheres for certain types of diving (diving air), and use in certain types of batteries, (e.g., pressurized nickel-hydrogen batteries), to give just a few examples.
Although it therefore has great utility, distribution of hydrogen has long been hampered by the difficulties inherent in storing and transporting it in gaseous form. When uncompressed (i.e., at atmospheric pressure) the gas simply occupies too much volume for practical use, and furthermore the gas generally needs to be under pressure for most uses. Storage in compressed form, however, requires use of pressure vessels of some form, which are typically heavy, bulky and dangerous to transport, as well as being relatively expensive. Typical are the ubiquitous high-pressure gas cylinders made of steel, commonly referred to as “K-cylinders.” These steel cylinders are notoriously heavy, cumbersome and difficult to transport, to the point where they are simply unsuitable for many applications that involve portability. The dangers that they present have also caused them to be prohibited from use in certain environments, for example, onboard certain naval vessels. Still further, conventional gas cylinders typically require valves and/or regulators to discharge the hydrogen at the required pressures/rates, which adds to complexity and cost. The cost of gas cylinders also makes it economically unviable to simply dispose of them after use, so that they must be transported back to a facility to be refilled at still additional cost. These sundry difficulties and expenses have the combined effect of rendering the use of hydrogen impractical in many circumstances where it would otherwise be beneficial.
An alternative to storing and shipping hydrogen as a pressurized gas is to generate it on location from chemical reactions using materials that can be stored/transported without needing pressure vessels. Water-hydride reactions (e.g., water+ lithium hydride) are perhaps the most well know, however the reactions are notoriously difficult to control, being rapid and highly exothermic, to the point of being potentially dangerous in some situations. Furthermore, disposal is a problem due to the potentially hazardous nature of the reaction products.
Hydrogen gas can also be produced using aluminum-based water-split reactions, which generally exhibit much more benign characteristics than hydride-based reactions. The reactions between aluminum and water (2Al+6H2O→2Al(OH)3+3H2↑; 2Al+4H2O→2AlO(OH)+3H2↑; 2AL+3H2O→Al2O3+3H2↑) are well known, but until recently their use in practical applications has been problematic due to the phenomenon known as “passivation”: Bare metallic aluminum almost immediately forms a very inert aluminum oxide layer on its surface that shields the underlying bulk aluminum and thereby inhibits further reactions between the aluminum metal and surrounding gases or liquids. A number of different approaches were previously developed in an effort to overcome the passivation problem, such as mechanically modifying particles of aluminum by milling or fracturing, but these have generally proven too energy-intensive and/or expensive to be economically viable. More recently though, as exemplified by the process disclosed in PCT Patent Application No. WO 2008/027524, it has been found that the passivation problem can be overcome using a water-soluble inorganic salt such as sodium chloride or potassium chloride as a “catalyst” that causes progressive pitting of the aluminum; These salts remove the passivation layer, through corrosive attack of the surface. In addition by adding certain metal oxides, such as calcium oxide or magnesium oxide, the reaction can be accelerated as a result of the heat generated when the metal oxides are exposed to water. The particles of metallic aluminum, salt catalyst and metal oxide initiator are (in the prior art) blended together into a homogenous, powder-like mix, to which water is added (or vice versa) to produce hydrogen when desired. The dry materials are safe and easy to store and transport, and the reaction products are substantially inert and environmentally benign and therefore can be readily disposed of almost regardless of location.
Although generally successful in overcoming the problem of passivation per se, the system described in the preceding paragraph is subject to inherent limitations that make it less than completely satisfactory for many applications. A particular problem involves the difficulty of adjusting or tailoring the speed or other characteristics of the reaction to the divergent requirements of different applications: For example, certain applications, such as filling balloons for meteorological or military applications, require that large volumes of hydrogen be produced in a very rapid manner. Other applications such as supplying hydrogen for use by a fuel cell, welding apparatus or other device normally call for a slower rate of production over a much longer period of time. Also, certain applications may call for production of heat/steam together with the hydrogen, whereas in other cases these products may be undesirable.
As compared with hydride-based reactions, the basic aluminum-catalyst-initiator system does offer greater controllability, but nevertheless with significant limitations. For example, the reaction may be controlled to a certain extent by metering the rate at which water is introduced to the blended material, while measuring pressure or otherwise monitoring the rate at which the hydrogen is produced; however, the metering and monitoring devices, such as valves, sensors, microprocessors, and so on, represent significant complexity, weight and expense, and moreover the rate of control that can be achieved in this manner is subject to certain practical limitations. Changing the proportions of the constituents (metallic aluminum-salt-catalyst-metal oxide initiator and metal hydroxide) in the particulate blend can also provide some degree of adjustability, but the range of adjustment that can be achieved in this manner is comparatively limited and inadequate to meet the requirements of many differing applications such as those discussed above.
Accordingly, there exists a need for methods and apparatus that can effectively produce hydrogen gas on location by chemical reaction, so as to obviate the distribution problems associated with use of compressed hydrogen gas. Furthermore, there exists a need for such methods and apparatus that permit the rate, temperature and other characteristics of the reaction to be configured or adjusted to meet the divergent needs of different applications. Still further, there exists a need for such methods and apparatus that can be configured or adjusted to produce heat and/or steam as products where desired. Still further, there exists a need for such methods and apparatus that make effective use of aluminum-based water-split reactions, so as to avoid the drawbacks inherent in hydride-based reactions and the like. Still further, there exists a need for such methods and apparatus that are economical in nature, can be conveniently and safely implemented in a wide variety of locations and conditions, and that present minimal costs and environmental/safety concerns relating to disposal of the expended materials.